The passage you quoted is emphasizing that we are born of a free woman Sarah and not a slave Agar. We are the spiritual children of Israel, and the children of promise. That freedom was purchased for us by Jesus Christ, but it was dependent on the “fiat” of that great and glorious second Eve, Mary, who is the mother of all those who are saved. Just as sin came to us through Eve, so also salvation came to us through Mary. Think of those great lines from Handel’s Messiah that speak of sin coming through the voluntary action of one man, and salvation coming to us through the act of the God Man:
“For as in Adam all die, even so in Christ shall all be made alive. “
There are great parallels there.
So then Catholics put Mary in the place of Sarah?